The extraction of fluids such as oil, gas or water from subterranean wells involves introducing a transportation pipe into the ground. The fluid is forced to the surface of the earth through the pipe by natural pressure in the well, a pump above ground, or displacing the fluid with another fluid, such as using water to displace oil. Such a process involves a flow of highly pressurised fluid into the pipe which inevitably carries along with it detritus in the form of sand, stones and other particles, which abrades and erodes the welling machinery. Therefore, it is a common practice to provide a filter assembly, known as a well screen, at the submerged opening of the transportation pipe to separate the fluid from the solids. The environment within the well is such that very high hydraulic pressure is applied to the well screen. Further, wells can be unstable, and so the well screen must withstand localised external loads from the partial collapse of the well wall.
In the construction of such well screens, it is common to have a layer of filter media to act as a filter against the introduction of detritus into the pipe. Typically the layer of filter material is applied to the base pipe as a sheet which wraps around the base pipe and seam welded. Such seam welding, however, can damage the filter media and so provide an area of weakness or localised failure proximate to the seam during adverse conditions.
Damage or misalignment to the filter layer can create an area whereby an inflow of detritus into the pipe can occur. Whilst the damage may be in a localised portion of the filter and so affecting only a small area of the pipe, because of the inflow of the detritus, the entire intake of the fluid may be contaminated. Accordingly, increased cost results, affecting the viability of the pipe as a whole. It will be appreciated that for seam welding, extended portions of the pipe may be affected or affected at multiple areas.
It follows that if the filter media is damaged and the inflow of detritus into the pipe is affecting performance, then a repair of the pipe will require the full extraction from the well leading to significant downtime. However, in most circumstances, a repair may not be possible as the filter media is typically encapsulated within an external shroud. If the shroud is not easily removable (and typically the shrouds of prior art well screens are not), then non destructive testing of the pipe will be required to determine the location of any damaged portions of the filter media. In many circumstances it may be easier to merely replace the well screen leading to a significant cost increase to the project.
Further, a failure in the filter media may lead to a significant drop in the pressure differential at that point. That is, the pressure drop associated with the filter media (if damaged), may be reduced to almost zero. Further, in the case of gaps in the filter media due to damage, the peripheral edges of this damaged zone may represent a stress concentration in the well screen. Such a stress concentration, if undetected may propagate the damage, increasing the potential for an influx of detritus as well as increasing the possibility of catastrophic collapse of the well screen and so increasing the overall cost of extraction of the failed well screen.